The surface of plated layer of a tin-plated steel strip produced by a commonly known continuous electrolytic tin-plating method is dull, and has no gloss. For the purpose of giving gloss to the surface of thus tin-plated layer, the method traditionally applied comprises heating said strip in a continuous heating furnace to cause rapid fusion of said tin-plated layer, and then immediately directing said strip into a quenching tank to quench it for solidification of the tin-plated layer.
If the quenching is improperly applied, however, dirt patterns which look like dried stains of dirty water (hereinafter referred to as "quench stain") are produced on the surface of tin-plated layer of the strip, considerably reducing the commercial value of the strip. Said quench stain is produced by an unevenly quenched tin-plated layer due to an irregular quenching rate caused by the non-uniform contact between the strip and the quenching liquid, which is brought about by surface turbulence of the quenching liquid in the quenching tank on introducing the strip into it, splashes of quenching liquid onto the strip, and uneven deposit of a steam film, which is generated by quenching, on the strip.
Also, said tin-plated layer of the strip produces an alloy layer of tin and iron at its fusion, and it is necessary to make it a uniform and dense alloy layer for obtaining an excellent corrosion resistance. For this purpose, there should be a certain duration of time sufficient to cause progressive development of the alloy layer. In other words, slowing down of the quenching rate of the strip is required to a certain degree. On the other hand, however, increasing the quenching rate is required for accelerating the quenching line operation.
In order to solve these problems, there have been a number of proposals. In a method disclosed in U.S. Pat. No. 3,358,980, for instance, two compartments with a hood and spaced plates are installed in a quenching tank, and surface turbulence on the surface of quenching liquid in the quenching tank caused by the incoming strip is prevented with the use of said spaced plates. Besides, a narrow region is confined by said spaced plates. The heated strip is directed into this narrow region. The quenching liquid first fed into said compartments flows down into the quenching tank by gravity along the both sides of the strip, in the same direction as the travel of the strip, and almost in parallel with the strip, in said narrow region, and after filling up the quenching tank, the quenching liquid overflows. Initially, the strip, brought into contact with the quenching liquid in said narrow region, is quenched at a relatively slow quenching rate, and secondly is moved into the quenching tank, being quenched down to a prescribed temperature. Further, a temperature sensing device is provided in said narrow region to control the quenching liquid temperature.
According to the above-mentioned method, no surface turblence is produced on the surface of the quenching liquid in the quenching tank on introducing the strip into it, so that the tin-plated layer is quenched uniformly, permitting prevention of quench stains. However, in this method, because the quenching liquid comes into contact with the strip while flowing down in said narrow region by gravity, the impact of the quenching liquid against the strip is small. Nevertheless, with the recent speeding-up of a continuous electrolytic tin-plating line, the moving speed of strip has been accelerated up to some 300-450 m/min. Accordingly, quenching capacity obtained only by a stream of quenching liquid by gravity as is the case of this method is insufficient. With an insufficient quenching capacity, steam generating at the interface between the strip and the quenching liquid accompanies the strip, being deposited on the surface of the strip. As a result, irregularity is found in the quenching rate of the tin-plated layer, which is not uniformly quenched, so that it is impossible to completely prevent the occurrence of quench stains. In other words, this method is not applicable to a high-speed continuous electrolytic tin-plating line. Besides, it is necessary to provide the quenching tank with a hood, spaced plates and compartments, leading to increased costs of quenching facilities.
Further, in a method disclosed in U.S. Pat. No. 3,410.734, an elongated conduit section of rectangular cross-section which provides a restricted quench channel extends upwardly from a quenching tank. A quenching liquid supplied into the quenching tank, after filling up the quenching tank, comes up in said restricted quench channel and flows over its upper end into a trough. Closely adjacent the upper end of said restricted quench channel, a plurality of submerged jet or spray units are provided for directing streams of quenching liquid toward a strip across the entire width thereof. A heated steel strip moves downwardly from a heating furnace and enters the restricted quench channel where it is immediately immersed in the upwardly flowing stream of quenching liquid. In addition, the submerged jet or spray units direct streams of quenching liquid against the strip in a direction generally normal to the strip. Said submerged jet or spray units use a large quantity of quenching liquid with a relatively low pressure of about 1.4 - 2.1 kg/cm.sup.2.
According to the above-mentioned method, the strip can be quenched over the entire width uniformly and at a high rate. Therefore, this method is applicable, in particular, for obtaining strips having martensitic microstructure and superior in flatness. However, this method has no special regard for the prevention of quench stains and for the achievement of an excellent corrosion resistance. Particularly, no regard is paid to such points as developing in the tin-plated layer an alloy layer necessary for obtaining an excellent corrosion resistance, in other words, making the quenching rate in the initial stage of quenching relatively slower. The method has another disadvantage of requiring a complicated quenching device which increases the installation costs.
There is also known a method to fuse electrolytically deposited tin in a heating furnace aiming at improving the corrosion resistance on either the upper or under surface of a continuously electrolytic tin-plated steel strip, then to spray quenching liquid on either the upper or under surface of the strip, in the air or beneath the surface of the quenching liquid in the quenching tank, and, by quenching while holding a temperature differential between both sides, to coarsen the grain size of tin on one side of the strip.
The method, though better in the slow quenching in the initial stage of quenching, has no regard to unfirom quenching of the strip over the entire width and quenching capacity. That is, in this method, increasing the pressure or the volume of water of submerged spray in the quenching tank in an attempt to improve the quenching capacity brings more serious surface turbulence of quenching liquid in the quenching tank without permitting uniform quenching of the strip over the entire width, resulting in the impossibility of preventing the production of quench stains. Moreover, quenching in the air by this method aims merely at controlling tin crystal, with no regard to uniform quenching of the strip over the entire width.
As described above, it is desired to obtain continuously electrolytic tin-plated steel strip excellent in corrosion resistance as well as glossy and with no quench stain, but a method and apparatus to obtain strip with these properties have not as yet been proposed.